Include/Exclude posts by tag name ?

Have a function to include only posts with a certain tag id (say 21) on the home page.

`function onlyinclude_tag($query) {
if ( $query->is_home) {
$query-> set(‘tag__in’,array(21));
}
return $query;
}
add_filter(‘pre_get_posts’,’onlyinclude_tag’);`

Would prefer if I could include by tag name instead tag id !

Any help much appreciated.

I think i am looking for something to identify the tag id from the name (could be wrong) like say

$tag_id = get_tag_ID('the name'); —–> made up –> but i’m looking for something like it ??

obviously there may be a simpler way!